1. Field of the Invention
The present invention relates to a method and apparatus for use in offshore drilling and well completion that prevents bending and buckling of marine drilling risers by supplying axial tension to the riser through controllably buoyant open-bottom float cans.
2. Description of the Prior Art
Various types of axially tensioned risers are commonly used. One such device is shown in U.S. Pat. No. 3,017,934, entitled "Casing Support," issued to A. D. Rhodes, et al., on January 23, 1962. In this patented riser, one embodiment has a plurality of float cans or buoyancy pods which decrease in available buoyant volume upwardly from the lowest float can. Each float can is made up of a cylinder open at its lower end with a bell cap at its upper end. The upper bell cap of each float can is connected to the riser in such a way as to provide a pressure-proof connection. A conduit connects the lower end of each float can with the lower end of the succeeding one above it.
A source of compressed air located on a floating vessel is connected to the conduit to supply air under pressure to the lower end of the lowermost float can and then to each succeeding one. The compressed air source is connected to the uppermost float can by a return line. Subsequent to anchoring this riser and after attaining the requisite tension, a continual or periodic introduction of air is provided to compensate for the loss of air which goes into solution with the water at the open-bottom end.
The foregoing arrangement has some inadequacies. Among them is the inability to adjustably control the tensioning of the riser since all the buoyancy pods are filed with compressed air from the bottommost pod to the topmost one. Additionally, this riser does not have a built-in safety feature that comes into operation if the riser should part resulting in uncontrolled movement and subsequent damage. The present invention, on the other hand, can be controllably tensioned. But most importantly, the present invention includes a built-in safety mechanism that can eliminate the costs of both repair and accompanying drilling down time due to damage from an uncontrolled parted riser. Also, the present invention, as will become evident, has the ability to be lowered without being ballasted.